Photoinduced transition from insulator to conductor

Abstract

Methods for inducing reversible or permanent electrical conductivity in wide bandgap metal oxides such as Ga2O3 using light rather than doping are described, as well as related compositions and devices.

Description

[Technical Field]

[0001] Related Applications This application claims priority under 35 U.S.C. §111(b) to U.S. Provisional Patent Application No. 62 / 970,349, filed February 5, 2020, the entire disclosure of which is incorporated herein by reference.

[0002] Statement of Federally Funded Research This invention was made without government support. The government has no rights in this invention. [Background technology]

[0003] Gallium oxide (Ga2O3) is a desirable material in many optoelectronic applications. However, even though such materials are highly desirable for certain types of devices, no method exists for producing electrically conductive Ga2O3 materials. It would be advantageous to develop a method for producing electrically conductive Ga2O3 materials. Summary of the Invention

[0004] A method for producing an electrically conductive material is provided, the method comprising exposing a wide bandgap metal oxide having oxygen vacancies to sub-bandgap light for a duration to induce electrical conductivity in the wide bandgap metal oxide and produce an electrically conductive metal oxide having either reversible or permanent electrical conductivity.

[0005] In certain embodiments, the wide bandgap metal oxide comprises Ga2O3. In certain embodiments, the method further comprises increasing the temperature of the wide bandgap metal oxide to an elevated temperature to eliminate the reversible electrical conductivity. In certain embodiments, the electrically conductive metal oxide comprises Ga2O3, which has reversible electrical conductivity. In certain embodiments, the electrically conductive metal oxide has reversible electrical conductivity and is present in an optical memory device. In certain embodiments, the sub-bandgap light is about 1.81×10 17photon / cm 2 The intensity is about 1 / 2 s and the duration is at least 1 hour. In certain embodiments, the wide bandgap metal oxide with induced electrical conductivity comprises n-type GaO having permanent electrical conductivity. In certain embodiments, the method further includes fabricating an electronic device using the n-type GaO.

[0006] In certain embodiments, the sub-bandgap light has an energy in the range of about 1.45 eV to about 3.39 eV, hi certain embodiments, the sub-bandgap light has an energy of about 3.1 eV.

[0007] In certain embodiments, the duration is in the range of about 1 minute to about 100 hours. In certain embodiments, the duration is in the range of about 1 hour to about 70 hours. In certain embodiments, the duration is at least about 1 hour. In certain embodiments, the duration is at least about 70 hours.

[0008] In one particular embodiment, the sub-bandgap light is about 1×10 15 photon / cm 2 ·Seconds ~ approx. 1×10 19 photon / cm 2 In one particular embodiment, the sub-bandgap light has an intensity in the range of about 1.81×10 17 photon / cm 2 · Has second strength.

[0009] Also provided are methods for producing electrical conductors, the methods comprising exposing an insulating material to light of sufficient energy and intensity for a sufficient time to induce permanent electrical conductivity in the insulating material, thereby producing the conductive material. In certain embodiments, the insulating material comprises a wide bandgap metal oxide having oxygen vacancies. In certain embodiments, the insulating material comprises Ga2O3 having oxygen vacancies. In certain embodiments, the insulating material is undoped. In certain embodiments, the method does not include doping the insulating material.

[0010] Also provided is a method for reversing the electrical conductivity of a material, the method comprising heating a wide bandgap metal oxide material having reversible electrical conductivity to an elevated temperature to substantially eliminate the electrical conductivity of the wide bandgap metal oxide material. In certain embodiments, the wide bandgap metal oxide material comprises oxygen vacancies. In certain embodiments, the wide bandgap metal oxide material comprises Ga2O3. In certain embodiments, the elevated temperature is at least about 36°C. In certain embodiments, the elevated temperature is at least about 100°C.

[0011] Also provided is a method for tailoring a material's properties, the method comprising providing a wide bandgap metal oxide material having oxygen vacancies; and exposing the wide bandgap metal oxide material to sub-bandgap light of a desired intensity for a desired time period, thereby inducing electrical conductivity in the wide bandgap metal oxide material and tailoring the electrical conductivity as desired. In certain embodiments, the wide bandgap metal oxide material comprises Ga2O3.

[0012] Also provided is a method for removing electrical conductivity from a material, the method comprising heating a wide bandgap metal oxide having permanent electrical conductivity to an elevated temperature in the presence of oxygen, thereby filling oxygen vacancies in the wide bandgap metal oxide and removing the electrical conductivity of the material, wherein the elevated temperature is at least about 800°C.

[0013] Also provided is a composition comprising GaO having reversible electrical conductivity, which decays over time and is substantially removable by heating the composition to elevated temperatures. In certain embodiments, the composition is undoped. Also provided is an optical memory device comprising the composition.

[0014] Additionally, compositions are provided that include undoped n-type Ga2O3, wherein the Ga2O3 has a 1.0 Ω -1 ·cm -1Also provided is an electronic device comprising this composition.

[0015] Additionally, an optical memory device is provided that includes a wide bandgap metal oxide having oxygen vacancies and reversible electrical conductivity, which can be reversibly restored by slightly increasing the temperature of the wide bandgap metal oxide.

[0016] The patent or patent application file may contain one or more drawings and / or one or more photographs executed in color. Copies of this patent or patent application publication with color drawings and / or photographs may be obtained from the U.S. Patent and Trademark Office upon request and payment of the necessary fee. [Brief explanation of the drawings]

[0017] [Figure 1A] Figures 1A-1C: Photoconductivity of undoped β-Ga2O3 bulk crystals. Figure 1A shows a schematic of the photoinduced transfer of electrons from localized states in the gap to the conduction band. [Figure 1B] Figures 1A-1C: Photoconductivity of undoped β-Ga2O3 bulk crystals. Figure 1B shows the dependence of photoconductivity on photoexcitation energy and intensity at room temperature. The initial dark electrical conductivity before irradiation with 2.69 eV photons was on the order of 106 ohm-1 cm-1, an order of magnitude larger than the initial values ​​in other measurements. This increase in dark electrical conductivity was due to exposure of the sample to room light. [Figure 1C] Figures 1A-1C: Photoconductivity of undoped β-Ga2O3 bulk crystals. Figure 1C shows the dependence of photoconductivity and photoinduced charge carrier density on photon energy at room temperature. [Figure 2] Figure 2: Change in electrical conductivity as a function of photon intensity in doped β-Ga2O3 bulk crystals: Fe-doped Ga2O3 and Mg-doped Ga2O3. In contrast to the doped crystals, the electrical conductivity in this case decreases upon exposing the sample to light, indicating that impurities such as Fe and Mg are not responsible for the induced photoconductivity of Ga2O3. [Figure 3A] Figures 3A-3D: Calculation of the decrease in electrical conductivity and charge carrier density with annealing temperature after photoexcitation, and the thermal barrier energy for electron recapture by defects. Figure 3A shows a diagram illustrating the experimental workflow. [Figure 3B] Figures 3A-3D: Decrease in electrical conductivity and charge carrier density with annealing temperature after photoexcitation, and calculation of the thermal barrier energy for electron recapture by defects. Figure 3B shows electrical conductivity and carrier density versus annealing temperature. [Figure 3C] Figures 3A-3D: Calculation of the decrease in electrical conductivity and charge carrier density with annealing temperature after photoexcitation, and the thermal barrier energy for electron recapture by defects. Figure 3C shows the logarithm of the carrier density versus 1 / kT, where k is Boltzmann's constant and T is temperature in Kelvin. [Figure 3D] Figures 3A-3D: Calculation of the decrease in electrical conductivity and charge carrier density with annealing temperature after photoexcitation, and the thermal barrier energy for electron recapture by defects. Figure 3D shows a schematic diagram of the excitation of electrons from defect states to the conduction band and the thermal barrier energy for electron recapture. [Figure 4] Figure 4: Electronic conductivity as a function of irradiation time (measured in situ under light). The inset shows a magnified view of the increase in electrical conductivity upon irradiation over the first few hours, showing saturation at approximately 40 minutes. [Figure 5A] 5A-5B: Photoconductivity decay as a function of time after photoexcitation is discontinued. Figure 5A shows the photoconductivity after exposing the sample to light of different lattice energies for 1 hour. [Figure 5B]Figures 5A-5B: Photoconductivity decay as a function of time after cessation of photoexcitation. Figure 5B shows the photoconductivity after exposing the sample to 3.1 eV photoexcitation. The blue curve represents the decay of electrical conductivity after irradiating the sample for 70 hours. After the initial decay of electrical conductivity, the sample was again exposed to a higher photon intensity for approximately 10 minutes (red curve). These results demonstrate the dependence of the rate of electrical conductivity decay on the energy, intensity, and time of photoexcitation, as well as on repeated exposure to light. [Figure 6] Figure 6: Changes in electronic conductivity in undoped β-Ga2O3 after exposure to photoexcitation and annealing. After repeated photoexcitation at 3.1 eV, the electrical conductivity increased by nine orders of magnitude from 10-8 Ω-1 cm-1 to nearly 1 Ω-1 cm-1 and remained at this conductivity without decay, indicating a transition from an insulating to a conducting state. Subsequent annealing of the sample at 800 °C for 2 h in O2 reduced the electrical conductivity to approximately 10-6 Ω-1 cm-1. Extended re-photoexcitation for 20 h after annealing did not increase the electrical conductivity. Conversely, the electrical conductivity further decreased, indicating that annealing at high temperatures in O2 caused the sample to lose all of its photoconductive features. [Figure 7A] 7A-7B: Electronic conduction versus temperature characteristics of the permanently conducting state in Ga2O3. Figure 7A shows the sheet resistance and sheet number as a function of temperature. [Figure 7B] Figures 7A-7B: Electronic conduction versus temperature characteristics of permanently conducting states in Ga2O3. Figure 7B shows the sheet number Ln as a function of the inverse temperature. The measurements reveal a frozen region of charge carriers, indicating that the induced new states are shallow states within the band gap rather than the conduction band. [Figure 8A] Figures 8A-8C: Internal structural arrangement of β-GaO. Figure 8A shows the undoped structure. The variation in the distance of Ga atoms from vacancies is also shown. The red and green arrows with numerical values ​​indicate the amount of Ga atom movement away from or towards the vacancy, respectively. The gray and orange spheres represent Ga and O atoms, respectively. [Figure 8B] Figures 8A-8C: Internal structural arrangement of β-GaO. Figure 8B shows the internal structural arrangement with neutral oxygen vacancies. The variation in the distance of Ga atoms from the vacancies is also shown. The red and green arrows with numerical values ​​indicate the amount of Ga atom movement away from or towards the vacancies, respectively. The gray and orange spheres represent Ga and O atoms, respectively. [Figure 8C] Figures 8A-8C: Internal structural arrangement of β-GaO. Figure 8C shows the internal structural arrangement with divalent oxygen vacancies. The variation in the distance of Ga atoms from the vacancies is also shown. The red and green arrows with numerical values ​​indicate the amount of Ga atom movement away from or towards the vacancies, respectively. The gray and orange spheres represent Ga and O atoms, respectively. [Figure 9A] Figures 9A-9C: 2D contour plots of the electron localization function (ELF) for undoped Ga2O3 (Figure 9A), Ga2O3 with neutral oxygen vacancies (Figure 9B), and Ga2O3 with oxygen vacancies with a 2+ charge (Figure 9C). Blue, green, and red colors correspond to ELF values ​​of 0, 0.5, and 1, respectively. [Figure 9B] Figures 9A-9C: 2D contour plots of the electron localization function (ELF) for undoped Ga2O3 (Figure 9A), Ga2O3 with neutral oxygen vacancies (Figure 9B), and Ga2O3 with oxygen vacancies with a 2+ charge (Figure 9C). Blue, green, and red colors correspond to ELF values ​​of 0, 0.5, and 1, respectively. [Figure 9C] Figures 9A-9C: 2D contour plots of the electron localization function (ELF) for undoped Ga2O3 (Figure 9A), Ga2O3 with neutral oxygen vacancies (Figure 9B), and Ga2O3 with oxygen vacancies with a 2+ charge (Figure 9C). Blue, green, and red colors correspond to ELF values ​​of 0, 0.5, and 1, respectively. [Figure 10A]10A-10C: Total (gray) and partial (line) electronic density of states (DOS) for undoped GaO (FIG. 10A), GaO with neutral oxygen vacancies (FIG. 10B), and GaO with charged oxygen vacancies (FIG. 10C). The partial DOS for s and p for Ga and p for oxygen are represented by the green, blue, and red lines, respectively. Zero energy is referenced to the highest occupied state of the system, referred to herein as the Fermi energy, EF. [Figure 10B] 10A-10C: Total (gray) and partial (line) electronic density of states (DOS) for undoped GaO (FIG. 10A), GaO with neutral oxygen vacancies (FIG. 10B), and GaO with charged oxygen vacancies (FIG. 10C). The partial DOS for s and p for Ga and p for oxygen are represented by the green, blue, and red lines, respectively. Zero energy is referenced to the highest occupied state of the system, referred to herein as the Fermi energy, EF. [Figure 10C] 10A-10C: Total (gray) and partial (line) electronic density of states (DOS) for undoped GaO (FIG. 10A), GaO with neutral oxygen vacancies (FIG. 10B), and GaO with charged oxygen vacancies (FIG. 10C). The partial DOS for s and p for Ga and p for oxygen are represented by the green, blue, and red lines, respectively. Zero energy is referenced to the highest occupied state of the system, referred to herein as the Fermi energy, EF. DETAILED DESCRIPTION OF THE INVENTION

[0018] Throughout this disclosure, various publications, patents, and published patent specifications are referenced by an identifying citation. The disclosures of these publications, patents, and published patent specifications in their entireties are incorporated by reference into this disclosure in order to more fully describe the state of the art to which this invention pertains.

[0019] According to the present disclosure, photoconductivity can be induced in certain wide-bandgap metal oxides with oxygen vacancies by exposure to light of a certain energy and intensity for a certain period of time. Two distinct effects are possible: reversible electrical conductivity and permanent electrical conductivity. Photoconductivity can be reversible, in which case electrical conductivity decays over time and can be reduced or eliminated by increasing the temperature of the material, or it can be permanent, in which case electrical conductivity does not decay over time and cannot be eliminated unless the material is heated to extremely high temperatures in the presence of oxygen. The present disclosure therefore provides for the conversion of an insulator to a conductor by applying light to the material.

[0020] The light used to induce photoconductivity can be provided by any light source, such as a light-emitting diode (LED), and is generally sub-bandgap light. In other words, the light has an energy lower than the bandgap of the material to be exposed to the light. When exposed to sub-bandgap light, electrons are pumped from defect levels into the conduction band. Without being bound by theory, it is believed that the lattice structure of the material contracts due to the removal of electrons from the defect levels, which causes a different distribution of electrons in the lattice structure. This shifts the energy levels of oxygen vacancies in the material, making them shallow donors.

[0021] The variables of light intensity and duration of light exposure (i.e., photoexcitation time, also referred to as irradiation time) work together to determine whether the resulting material has reversible or permanent electrical conductivity. If the exposure time is short and the intensity is low, the electrical conductivity is reversible. If the exposure time is long and the intensity is high, the electrical conductivity is permanent. Permanent electrical conductivity can also be achieved by using high intensity with short exposure times, and by using long exposure times with low intensity. Light intensity is about 1×10 15 photon / cm 2 ·Seconds ~ approx. 1×10 19 photon / cm 2In one non-limiting example, the light intensity may be in the range of about 1.81×10 17 photon / cm 2 10 sec. The photoexcitation time may be in the range of about 1 minute to about 150 hours, or about 30 minutes to about 100 hours, or about 1 hour to about 70 hours. In some embodiments, the photoexcitation time is at least about 1 hour. In some embodiments, the photoexcitation time is at least about 70 hours. In one non-limiting example, the light intensity is about 1.81×10 17 photon / cm 2 10 seconds and the irradiation time is about 1 hour.

[0022] While the present disclosure is not limited to any one particular material and, in fact, may be applicable to any wide bandgap metal oxide with oxygen vacancies, GaO is described herein for illustrative purposes. GaO with oxygen vacancies can be grown, for example, by using oxide-rich conditions.

[0023] In a first aspect, a GaO single crystal with oxygen vacancies can be exposed to sub-bandgap light for a limited time to create electrical conductivity. Exposing a GaO single crystal with oxygen vacancies to sub-bandgap light for a limited time, for example, by limiting the excitation time or reducing the photon intensity, can create reversible electrical conductivity. This reversible electrical conductivity can then be erased by increasing the temperature slightly above room temperature, as shown and confirmed in Figures 3A-3D. This method can be used to develop optical memory devices. Thus, further provided herein are compositions and devices (e.g., optical memory devices) comprising wide-bandgap metal oxides, such as undoped GaO, that have reversible electrical conductivity, where the reversible electrical conductivity can be removed by increasing the temperature of the wide-bandgap metal oxide.

[0024] Measurements described in the examples herein also demonstrate that irradiating undoped GaO with sub-bandgap light for extended periods of time results in a permanent, irreversible transition from a highly insulating state to an electrically conductive state, i.e., permanent electrical conductivity is created. In other words, light can be used to induce permanent electrical conductivity in wide-bandgap metal oxides such as GaO, without doping, converting the material from an insulator to a conductor. To achieve this, undoped GaO single crystals with oxygen vacancies can be exposed to visible light, controlling the light energy and intensity, as well as the exposure time. Thus, in a second embodiment, GaO single crystals with oxygen vacancies can be exposed to high-intensity sub-bandgap light for extended periods of time (Figures 4-6), followed by terminating the light, resulting in undoped n-type GaO semiconductor material. This method can be used in electronics development.

[0025] β-Ga2O3 is a highly resistive semiconducting oxide due to its wide bandgap. It is highly unique and surprising that a highly insulating material can be transformed into an electrically conductive material simply by exposing the material to light for a duration of time. In the examples herein, this phenomenon is demonstrated with Ga2O3, a material that is highly useful in various types of devices. The permanent transition from a highly insulating state to an electrically conductive state that is irreversible (except at extremely high temperatures, such as 800°C in the presence of oxygen, as seen in Figure 6) is a unique and surprising phenomenon with significant implications for both the properties of the material and potential applications. This phenomenon can be used in connection with various n-type Ga2O3-based devices and technologies, including, for example, optical memory devices and electronics, in which Ga2O3 functions as a semiconductor. Accordingly, compositions and electronic devices comprising n-type Ga2O3 are also provided herein, where the n-type Ga2O3 is undoped and has a resistivity of 1.0 Ω. -1 ·cm -1 It has an electrical conductivity of the order of .

[0026] The dependence of electrical conductivity decay on the duration and intensity of photoexcitation provides a means to tailor material properties and develop optically controllable devices. In particular, as one non-limiting example, optical memory devices can be fabricated that rely on the induction and removal of photoconductivity in materials such as Ga2O3. Optical memory devices rely on the storage of data on an optically readable medium. Data can be recorded by creating a pattern of marks that can be read back with the aid of light, such as a precisely focused laser beam, on a rotating optical disk. An optical memory device can include a wide-bandgap metal oxide material with oxygen vacancies, such as Ga2O3, a light source (e.g., a laser) for exposing the material to light to induce reversible electrical conductivity in the material, and an energy source (e.g., the same or a different laser) for increasing the temperature of the material to substantially remove the electrical conductivity.

[0027] Example example The transition from an insulator to a conductor, achievable in some materials, involves altering both the arrangement of atoms and their electronic configuration. This is often achieved by doping. These examples illustrate the lattice mechanism used to induce such a transition. Limited, long-duration exposure to subbandgap light is shown to induce a permanent transition from an insulator state to a conductor state in the insulating oxide Ga2O3, accompanied by a nine-order increase in electronic conductivity. The underlying mechanism is shown to be that photoexcitation alters the charge state of O vacancies and the redistribution of localized electrons, thereby severely distorting the structure of the Ga2O3 lattice. This alters the density of states and induces new stable states at shallower energy levels, leading to this intriguing behavior. This mechanism can also occur in other wide-bandgap metal oxides, leading to dramatic changes in their electronic properties.

[0028] When light strikes a semiconductor material, charge carriers, electrons and holes, can be generated, resulting in improved electrical conductivity. If the energy of the incident photon is greater than the band gap of the semiconductor, it excites electrons from the valence band to the conduction band, a phenomenon known as intrinsic photoconduction. If the energy of the incident photon is less than the band gap, it excites electrons from defect levels to the conduction band, enhancing electrical conductivity. This is known as extrinsic photoconduction. In either case, if electrical conductivity persists after photoexcitation ceases, it is known as persistent photoconduction. In this situation, when electron-hole pairs are generated, a microscopic or macroscopic potential barrier must exist to separate the charge carriers, reducing their likelihood of recombination and resulting in enhanced electrical conductivity over a longer period of time.

[0029] Persistent photoconductivity at room temperature has been reported primarily in semiconductor heterostructures and a few bulk materials. These examples demonstrate extrinsic persistent photoconductivity (also called reversible electrical conductivity) behavior in bulk Ga2O3 and a surprising permanent transition from an insulating state to a conducting state upon exposure to sub-bandgap light for a limited time. First, Ga2O3 bulk crystals exhibited strong persistent photoconductivity upon exposure to sub-bandgap light with energies much lower than the bandgap. Second, the photoinduced metastable state became stable only upon prolonged photoexcitation, resulting in a permanent transition from an insulating state to a conducting state. Such behavior has significant implications for material properties and applications.

[0030] Ga2O3 is the widest bandgap transparent (up to the UV-C range) semiconducting oxide known to date. Its extremely wide bandgap (approximately 4.5-4.9 eV) can lead to unique electronic phenomena. Due to this wide bandgap, UV-C transparency, and excellent thermal and chemical stability, it has numerous potential applications in power and high-voltage devices, Schottky diodes, field-effect transistors, gas sensors, phosphors and electroluminescent devices, UV photodetectors, and more. Ga2O3 exhibits polymorphs designated α, β, γ, and δ, with β-Ga2O3 being the most stable phase from room temperature to its melting point. As the most stable form, β-Ga2O3 is also the most studied polymorph. It crystallizes in a monoclinic structure with space group C2 / m and lattice parameters a = 12.2140□, b = 3.03719□, c = 5.7819□, and β = 103.83°. It has equal numbers of both octahedral and tetrahedral cationic sites. Ga2O3 is an insulator at room temperature due to its wide band gap, but electronic conduction has been reported when synthesized under reducing conditions. Theoretical calculations indicate that oxygen vacancies are deep states and cannot provide conduction electrons. It has also been proposed that silicon, the major impurity in Ga2O3, may be responsible for the conduction electrons. No effective hole states have been reported in Ga2O3, and theoretical calculations have shown that the valence band is flat, indicating that the effective mass for holes is larger, making p-type conduction difficult.

[0031] Results and Discussion Ga2O3 single crystals were irradiated with sub-bandgap light, and the electrical conductivity and carrier density were measured during irradiation. Figures 1A–1C show the dependence of photoconductivity on photoexcitation energy and intensity in undoped β-Ga2O3 single crystals. In Figure 1B, photoconductivity is plotted against light intensity at photon energies of 3.39, 3.22, 3.1, and 2.69 eV. It can be seen that the electrical conductivity increases sharply at low photon intensities and quickly saturates. In Figure 1C, photoconductivity and photoinduced charge carrier density are plotted as a function of photon energy. Before each measurement, the sample was heated to 400 °C for 1 h to maintain its initial dark electrical conductivity, and excitation was performed at the same photon intensity for all energies. Interestingly, all sub-bandgap photoexcitations resulted in an increase in electrical conductivity, even at energies as low as 1.9 and 1.45 eV. The maximum photoconductivity was obtained at 3.1 eV, which is much lower than the bandgap energy of Ga2O3, 3.5 / 3.9 eV. The increase in electrical conductivity due to sub-bandgap photoexcitation can be explained by the excitation of electrons from localized states in the gap to the conduction band, as shown in Figure 1A. In contrast to undoped Ga2O3, the Fe-doped and Mg-doped samples exhibited different behavior upon exposure to light. Both the Fe-doped and Mg-doped crystals exhibited a decrease in electrical conductivity upon exposure to 400 nm and 365 nm light (Figure 2). This indicates that common impurities in Ga2O3, such as Fe, are not responsible for this photoconductivity.

[0032] After terminating the photoexcitation, the sample exhibits significant persistent photoconductivity when irradiated with 3.1 eV. To calculate the associated potential barrier that prevents charge carriers from being recaptured by trapping centers after the photoexcitation is stopped and is therefore the largest source of persistent photoconductivity, the photoconductive sample was annealed inside a Hall effect measurement chamber at various temperatures (300°C to 390°C) for 10 min at each temperature. After each anneal, the sample was subsequently cooled to room temperature, and the electrical conductivity and carrier density were measured. The steps of the experimental procedure are shown in Figure 3A. Figure 3B shows how the sample's electrical conductivity and charge carrier density vary with annealing temperature, and Figure 3C shows the corresponding plot of the natural logarithm of the charge carrier density versus the reciprocal of the thermal energy (1 / kT). Using the Arrhenius equation, n = Ae - Eth / kT, the slope of the best-fit line yields a thermal energy barrier of approximately 0.157 ± 0.04 eV. Figure 3D illustrates the process of electron pumping from a localized center into the conduction band, where the center relaxes to a metastable state, and the subsequent process of electron recapture through the barrier energy Eth. However, subsequent repeated photoexcitation over a long period of time resulted in stable (permanent) electronic conduction that did not decay even after heating up to 400 °C. The decay of the induced electrical conductivity and its dependence on photon energy, intensity, and irradiation time are discussed in detail below.

[0033] Figure 4 shows the electrical conductivity of the sample as a function of irradiation time during extended exposure to 400 nm (3.1 eV) excitation. The conductivity initially increased rapidly and remained nearly constant after 40 minutes of exposure. The exposure continued for 70 hours and then stopped. Figure 5A shows the decay of electrical conductivity over time after the light was turned off after 1 hour of excitation. For 3.39 and 3.22 eV excitation, a drop in electronic conductance was observed immediately after the light was turned off. However, for 3.1 eV excitation, the conductivity decreased gradually, reaching the dark conductivity value after 8 minutes. This indicates that there are more than two localized states in the band gap, and that 3.1 eV light most likely excites electrons from centers exhibiting metastable states with longer decay times. Figure 5B (blue curve) shows the decay of electrical conductivity as a function of time after 70 hours of exposure to 3.1 eV excitation. After decay, the sample was then re-exposed to 3.1 eV photons for 10 minutes. The red curve in Figure 5B represents the subsequent decay of electrical conductivity. Each decay curve in Figure 5B exhibits two relatively fast and slow time decay constants. The faster conductivity decay rate was 0.4 Ω after 1 hour of excitation. -1 ·cm -1 / min, 0.008Ω after 70 hours of excitation -1 ·cm -1 / min, and after repeated photoexcitation at higher intensities, 0.004 Ω -1 ·cm -1 / min. The dependence of the photoconductivity decay on the excitation time is peculiar. However, these measurements clearly show a strong dependence of the electrical conductivity decay rate on the energy, intensity, and time of photoexcitation. It is interesting to note that repeated exposure to light has a significant effect on the electrical conductivity decay, leading to more stable electronic conduction.

[0034] To further explore the conditions that cause a permanent transition from the insulating to the conducting state, an undoped Ga2O3 sample was exposed to light for several times and the electrical conductivity was monitored. The initial electrical conductivity was 1.08 × 10 -8 Ω -1 ·cm -1Upon exposure to 3.1 eV photoexcitation, the electrical conductivity immediately increased by almost two orders of magnitude, but remained at nearly the same initial value after the light was turned off. However, with repeated photoexcitation and after prolonged exposure to light, the electrical conductivity increased by nine orders of magnitude and remained undetermined after the light was turned off, indicating a complete conversion from the insulating to the conducting state. Annealing the sample at 400 °C for 1 hour in the dark did not eliminate or reduce the electrical conductivity. The sample was converted to an electrical conductivity of 7.69 × 10 -7 Ω -1 ·cm -1 A much higher annealing at 800 °C for 2 hours in flowing O2 was required to convert the sample back to an insulator. However, this annealing also completely eliminated the photoconductive features of the sample. These results are summarized in Figure 6.

[0035] Defects are thought to provide localized states in the band gap, resulting in persistent photoconductivity. The unique permanent insulator-to-conductor transformation observed here, and the ability to eliminate this effect by annealing in O2 at high temperatures, confirm the significant role of defects. To investigate the presence and nature of defects in Ga2O3 samples, we performed positron annihilation spectroscopy (PAS), a well-established technique for studying vacancy-type defects. Positron annihilation lifetime spectroscopy (PALS) was performed using gamma-ray-induced positron spectroscopy (GIPS) at the ELBE (Electron Linac with high Brilliance and low Emittance) facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Dresden, Germany. GIPS is an advanced PAS technique capable of generating positron decay curves free of background or source contributions. It uses high-energy gamma rays to generate positrons directly inside the sample by pair production. Its main advantage is that unwanted contributions from the source or cladding material are completely eliminated from the positron annihilation, thus resulting in an accurate measurement of the positron lifetime. PALS measurements on the undoped Ga2O3 sample used in these examples reveal a lifetime of 187 ± 1 ps. This relatively short lifetime cannot be associated with Ga vacancies, which strongly trap positrons and result in a much longer lifetime. Compared to the reported bulk positron lifetime of 175 ps in Ga2O3, the measured 187 ps here is somewhat longer than the bulk positron lifetime, a result that often indicates the presence of oxygen vacancies in the oxide.

[0036] Based on PAS measurements and oxygen annealing experiments, the persistent photoconductivity in these undoped Ga2O3 crystals is associated with the presence of a high concentration of oxygen vacancies. These results are explained as follows: oxygen vacancies, VO, which are in a neutral charge state, form localized occupied deep states in the band gap, which do not lead to electrical conductivity. By exposing the samples to sub-bandgap light, electrons are pumped into the conduction band via two excitation steps, resulting in the formation of VO 2+ states, which can lead to shallow states. Indeed, the temperature dependence of the induced electrical conductivity and electron density presented in Figure 7 shows regions of frozen electrons, indicating that these new states are still within the band gap.

[0037] To understand the reasons behind the permanent transformation from an insulator to a conductor and to clarify the mechanism by which electrons are prevented from returning to their centers after terminating light, we investigated the changes in the structural properties of β-Ga2O3 using first-principles electronic structure calculations. Two different types of Ga sites exist in the crystal structure of β-Ga2O3. The first is Ga coordinated by four oxygen atoms (referred to as Ga1), while the second is Ga coordinated by six oxygen atoms (referred to as Ga2, as shown in Figure 8). The structure also has three non-equivalent oxygen sites: two tricoordinate O(I) and O(II) sites, and one tetracoordinate O(III) site. Of these three different sites, the O(II) site has the lowest formation energy for neutral oxygen vacancies. Therefore, in these examples, we focused on O(II)-type vacancies to evaluate the structural distortion caused by the net charge on the defect. In the presence of neutral oxygen vacancies (VOX) and charged oxygen vacancies (VO...), Ga triangles (formed by a vacancy surrounded by three Ga atoms, shown by the black dotted lines in Figure 8) were observed to contract and expand, respectively, as the net charge on the vacancy went from neutral to positive. The average Ga-Ga bond lengths were 3.129□ and 4.311□ for the neutral and charged cases, respectively (for comparison, the average Ga-Ga bond length in the undoped (defect-free) structure was 3.306□). More specifically, in the case of VOX, two Ga1-type atoms relax toward the vacancy site, while the Ga2-type atom relaxes away from the vacancy. In contrast, in the case of VO..., both the Ga1-type and Ga2-type atoms relax away from the vacancy. The variation in the distance of the Ga atoms from the vacancy compared to the undoped structure is shown in Figure 8. Therefore, relaxation inwards and outwards of the Ga triangles decreases (in the case of VOX) and increases (in the case of VO..) the average Ga-Ga bond length compared to the undoped case.

[0038] To probe the structural distortion around the vacancy, we analyzed the electron localization function (ELF) in the three systems. The ELF directly describes the electron distribution in space, which is useful for investigating bonding features. The local value of the ELF at a position can be interpreted as the probability of finding an electron at that position, given the presence of nearby electrons.

[0039] ELF values ​​range from 0 to 1. ELF values ​​close to 1 indicate spatial regions where electron localization is likely to occur, while values ​​of zero correspond to regions where electrons are completely delocalized or absent. ELF values ​​close to half indicate regions that exhibit electron gas-like behavior. Figure 9 shows two-dimensional ELF contour plots of undoped Ga2O3 and Ga2O3 with neutral and charged vacancies. While the electron localization on Ga is lower than on O, there is strong overlap. The potential for electron localization redistribution in the case of a neutral O vacancy is shown in Figure 9B, where a strong electron localization probability is observed between two Ga atoms close to the vacancy site. This shortens the Ga1-Ga2 bond length, pushing the Ga1 toward the defect site. The Ga2 atom also interacts slightly with this localized charged state, but its interaction with the neighboring O is much stronger, which pulls the Ga2 away from the defect site. In the case of a charged vacancy (Figure 9C), we observe an ELF very similar to that of the undoped case, except around the defect site. Furthermore, the ELF does not show a localized charge state between the two Ga1 atoms. Rather, the Ga1 atom has moved from its original position, away from the defect site, forming a new Ga1-O bond. However, the ELF of the Ga2 atom is similar to that of the neutral vacancy. This results in a significant expansion of the Ga triangle, with a corresponding increase in the lengths of the Ga1-Ga1 and Ga1-Ga2 bonds.

[0040] To gain insight into the effect of the structural relaxation of neutral and charged vacancies on the electronic states within the material, the total and fractional electron density of states (DOS) are plotted and shown in Figure 10. The DOS for the undoped structure is also shown for comparison. In Figure 10, it is observed that the presence of neutral vacancies induces occupied localized states just above the valence band due to the internal inward relaxation shown in Figure 8B. On the other hand, in the case of charged oxygen vacancies, the outward relaxation of the Ga triangle shifts the defect states to higher energies, closer to the lowest region of the conduction band (the DOS in Figure 10 shows that they overlap with the conduction band to an extent that they are difficult to distinguish with the naked eye). These states are now unoccupied.

[0041] These calculations show that the change in the vacancy charge state leads to strong structural relaxation and a change in the defect state in the band gap. This confirms the experimental scenario of a change in vacancy structure upon excitation. The change in vacancy charge state not only empties the defect state, but also significantly shifts it toward the conduction band, creating a state where there is no energetic driving force for the excited electron to reunite with the vacancy.

[0042] conclusion These examples reveal the mechanism of insulator-conductor transition through the redistribution of electron localization within the lattice induced by changing the charge state of defect centers, followed by dramatic lattice distortion and a large shift in the density of states. Measurements show that irradiating undoped Ga2O3 with sub-bandgap light for a limited time results in a permanent transition from a highly resistive to an electrically conductive state that is irreversible (except at extremely high temperatures in the presence of oxygen), a unique and surprising phenomenon with significant implications for both the material's properties and potential applications. Such a mechanism may also occur in other wide-bandgap oxides, potentially impacting their properties and applications. The dependence of the electrical conductivity decay on the photoexcitation time and intensity revealed in these examples opens new areas for tailoring material properties and developing optically controllable devices. By limiting the excitation time or reducing the photon intensity, electrical conductivity can be generated and erased, providing applicability for optical memory applications. Long-term excitation can also be used to develop n-type semiconductors for electronics.

[0043] method Undoped, Fe-doped, and Mg-doped Ga2O3 bulk crystals grown by the Czochralski (CZ) method were obtained from Synopsis Inc. The as-grown crystals were approximately 1 cm in diameter and sliced ​​into 1 mm-thick pieces. The electrical transport properties of the samples were measured using an MMR Hall effect measurement system. Prior to the measurements, the samples were properly cleaned, and indium contacts were placed on the surface of each sample. Light-emitting diodes (LEDs) of various wavelengths (365, 385, 400, 460, 650, and 850 nm) were used to provide optical excitation of 3.39, 3.22, 3.1, 2.69, 1.9, and 1.45 eV, respectively, and optical Hall measurements were performed at room temperature. The intensity of the optical excitation was varied by changing the current passing through the LED. For optical Hall measurements, a Hall effect measurement chamber was customized with a transparent window for sample illumination, and a Joule-Thomson refrigerator was used to maintain a constant sample temperature and overcome the heating effects caused by light illumination. The refrigerator was operated by passing high-pressure nitrogen gas through a thin pipe. Because light induces heat, this setup is important for photoconductivity experiments, which investigate carrier concentration and electrical conductivity due solely to photoexcitation without the influence of thermal contributions. Temperature-dependent Hall effect measurements were performed on conductive samples in a permanent state between 10 and 300 K using a cryostat equipped with a He compressor.

[0044] Density functional theory calculations The structural properties of β-Ga2O3 were investigated using the mathematical formalism of density functional theory (DFT) implemented in VASP (Vienna Ab initio Simulation Package). The core electron behavior and the interaction of valence electrons with ions were represented by the projector augmented wave (PAW) method. The Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA) was used as the exchange-correlation functional to obtain the optimized ground-state structure. The Brillouin zone was sampled using 3 × 3 × 3 and 7 × 7 × 7 Monkhorst-Pack k-point meshes for the optimization and electronic structure calculations, respectively. The valence electrons were represented in a plane-wave basis set with a convergence energy cutoff of 520 eV. A supercell of 160 atoms (32 formula units) was considered in the calculations. The structure was optimized until the calculated Hellmann-Feynman forces were smaller than 0.0001 eV Å-1.

[0045] Certain embodiments of the compositions and methods disclosed herein are made clear in the above examples. It should be understood that these examples, while indicating particular embodiments of the present invention, are given by way of illustration only. From the above discussion and these examples, one skilled in the art will be able to ascertain the essential features of the present disclosure and will be able to make various changes and modifications to adapt the compositions and methods described herein to various uses and conditions without departing from the spirit and scope of the present disclosure. Various modifications may be made, and equivalents may be substituted for elements of the present disclosure without departing from the essential scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope of the present disclosure. [1] A method for producing an electrically conductive material, comprising exposing a wide bandgap metal oxide having oxygen vacancies to sub-bandgap light for a duration to induce electrical conductivity in the wide bandgap metal oxide, producing an electrically conductive metal oxide having either reversible or permanent electrical conductivity. [2] The method according to [1], wherein the wide band gap metal oxide comprises Ga2O3. [3] The method of [1], further comprising increasing the temperature of the wide bandgap metal oxide to an elevated temperature to eliminate the reversible electrical conductivity. [4] The method according to [1], wherein the electrically conductive metal oxide has reversible electrical conductivity and is present in an optical memory device. [5] The sub-bandgap light is about 1.81 × 10 17 photon / cm 2 The method according to [1], wherein the intensity is 0.5-1.5 seconds and the duration is at least 1 hour. [6] The method of [5], wherein the wide bandgap metal oxide having induced electrical conductivity comprises n-type Ga2O3 having permanent electrical conductivity. [7] The method according to [6], further comprising fabricating an electronic device using the n-type Ga2O3. [8] The method according to [1], wherein the sub-bandgap light has an energy in the range of about 1.45 eV to about 3.39 eV. [9] The method according to [1], wherein the sub-bandgap light has an energy of approximately 3.1 eV.

[10] The method of [1], wherein the duration is within a range of about 1 minute to about 100 hours.

[11] The method of [1], wherein the duration is within a range of about 1 hour to about 70 hours.

[12] The method of [1], wherein the duration is at least about 1 hour.

[13] The method of [1], wherein the duration is at least about 70 hours.

[14] The sub-bandgap light is about 1×10 15 photon / cm 2 ·Seconds to approx. 1×10 19 photon / cm 2 The method according to [1], having an intensity in the range of .sigma.

[15] The sub-bandgap light is about 1.81 × 10 17 photon / cm 2 The method according to [1], having an intensity of .sigma.

[16] A method of producing an electrical conductor, comprising exposing an insulating material to light of sufficient energy and intensity for a sufficient time to induce permanent electrical conductivity in the insulating material, thereby producing the electrical conductor material.

[17] The method of

[16] , wherein the insulating material comprises a wide bandgap metal oxide having oxygen vacancies.

[18] The method of

[16] , wherein the insulating material comprises Ga2O3 having oxygen vacancies.

[19] The method of

[16] , wherein the insulating material is undoped.

[20] The method according to

[16] , which does not include doping the insulating material.

[21] A method of reversing the electrical conductivity of a material, comprising heating a wide bandgap metal oxide material having reversible electrical conductivity to an elevated temperature to substantially eliminate the electrical conductivity of the wide bandgap metal oxide material.

[22] The method of

[21] , wherein the wide bandgap metal oxide material contains oxygen vacancies.

[23] The method of

[22] , wherein the wide bandgap metal oxide material comprises Ga2O3.

[24] The method according to

[21] , wherein the elevated temperature is at least about 36°C.

[25] The method according to

[21] , wherein the elevated temperature is at least about 100°C.

[26] A method for adjusting a property of a material, comprising: Providing a wide bandgap metal oxide material having oxygen vacancies; and exposing the wide bandgap metal oxide material to sub-bandgap light of a desired intensity for a desired time period, thereby inducing electrical conductivity in the wide bandgap metal oxide material and tailoring the electrical conductivity as desired; A method comprising:

[27] The method of

[26] , wherein the wide bandgap metal oxide material comprises Ga2O3.

[28] A method for removing electrical conductivity of a material, the method comprising heating a wide bandgap metal oxide having permanent electrical conductivity to an elevated temperature in the presence of oxygen, thereby filling oxygen vacancies in the wide bandgap metal oxide and removing the electrical conductivity of the material, wherein the elevated temperature is at least about 800°C.

[29] A composition comprising Ga2O3 having reversible electrical conductivity, wherein the reversible electrical conductivity decays over time and is substantially removable by heating the composition to an elevated temperature.

[30] The composition according to

[29] , wherein the composition is undoped.

[31] An optical memory device comprising the composition according to

[29] .

[32] A composition comprising undoped n-type Ga2O3, wherein the Ga2O3 has a resistivity of 1.0 Ω -1 ·cm -1 The composition has an electrical conductivity on the order of .

[33] An electronic device comprising the composition according to

[32] .

[34] An optical memory device comprising a wide bandgap metal oxide having oxygen vacancies and reversible electrical conductivity, wherein the reversible electrical conductivity can be reversibly restored by slightly increasing the temperature of the wide bandgap metal oxide.

Claims

1. 1. A method for producing an electrically conductive material, comprising exposing a wide bandgap metal oxide having oxygen vacancies to sub-bandgap light for a duration to induce electrical conductivity in the wide bandgap metal oxide, producing an electrically conductive metal oxide having either reversible or permanent electrical conductivity; wherein the wide band gap metal oxide includes Ga 2 O 3 ; method.

2. 10. The method of claim 1, further comprising increasing the temperature of the wide bandgap metal oxide to at least 800°C to eliminate the reversible electrical conductivity.

3. 10. The method of claim 1, wherein the electrically conductive metal oxide has reversible electrical conductivity and is present in an optical memory device.

4. The sub-bandgap light is 1.81×10 １７ photon / cm ２ 2. The method of claim 1, wherein the intensity is in the range of 1000 to 10 ... seconds and the duration is at least 1 hour.

5. The wide band gap metal oxide having induced electrical conductivity is a permanently conductive n-type Ga ２ O ３ The method of claim 4, comprising:

6. The n-type Ga ２ O ３ 6. The method of claim 5, further comprising fabricating an electronic device using

7. The method of claim 1 , wherein the sub-bandgap light is at an energy in the range of 1.45 eV to 3.39 eV.

8. The method of claim 1 , wherein the sub-bandgap light is at an energy of 3.1 eV.

9. 2. The method of claim 1, wherein the duration is in the range of 1 minute to 100 hours.

10. 10. The method of claim 1, wherein the duration is in the range of 1 hour to 70 hours.

11. The method of claim 1 , wherein the duration is at least 1 hour.

12. 10. The method of claim 1, wherein the duration is at least 70 hours.

13. The sub-bandgap light is 1×10 １５ photon / cm ２ ・Seconds to 1×10 １９ photon / cm ２ The method of claim 1, having an intensity in the range of seconds.

14. The sub-bandgap light is 1.81×10 １７ photon / cm ２ The method of claim 1, having an intensity of seconds.

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